DATAJDBC-357 - Polishing.

Implemented workaround for MySql not supporting offset without Limit.
Using `SELECT 1` as dummy order by since it is documented to be optimized away.
Renamed tests to match the project standard.

See also:

- https://stackoverflow.com/a/44106422
- https://stackoverflow.com/a/271650

Original pull request: #125.

spring-data-relational/src/main/java/org/springframework/data/relational/core/dialect/LimitClause.java

@@ -19,6 +19,7 @@ package org.springframework.data.relational.core.dialect;
  * A clause representing Dialect-specific {@code LIMIT}.
  *
  * @author Mark Paluch
+ * @author Jens Schauder
  * @since 1.1
  */
 public interface LimitClause {
@@ -26,7 +27,7 @@ public interface LimitClause {
 	/**
 	 * Returns the {@code LIMIT} clause to limit results.
 	 *
-	 * @param limit the actual limit to use.
+	 * @param limit the maximum number of lines returned when the resulting SQL snippet is used.
 	 * @return rendered limit clause.
 	 * @see #getLimitOffset(long, long)
 	 */
@@ -35,19 +36,21 @@ public interface LimitClause {
 	/**
 	 * Returns the {@code OFFSET} clause to consume rows at a given offset.
 	 *
-	 * @param limit the actual limit to use.
-	 * @return rendered limit clause.
+	 * @param offset the numbers of rows that get skipped when the resulting SQL snippet is used.
+	 * @return rendered offset clause.
 	 * @see #getLimitOffset(long, long)
 	 */
-	String getOffset(long limit);
+	String getOffset(long offset);
 
 	/**
 	 * Returns a combined {@code LIMIT/OFFSET} clause that limits results and starts consumption at the given
 	 * {@code offset}.
 	 *
-	 * @param limit the actual limit to use.
-	 * @param offset the offset to start from.
+	 * @param limit  the maximum number of lines returned when the resulting SQL snippet is used.
+	 * @param offset the numbers of rows that get skipped when the resulting SQL snippet is used.
 	 * @return rendered limit clause.
+	 * @see #getLimit(long)
+	 * @see #getOffset(long)
 	 */
 	String getLimitOffset(long limit, long offset);
 

spring-data-relational/src/main/java/org/springframework/data/relational/core/dialect/MySqlDialect.java

@@ -19,6 +19,7 @@ package org.springframework.data.relational.core.dialect;
  * An SQL dialect for MySQL.
  *
  * @author Mark Paluch
+ * @author Jens Schauder
  * @since 1.1
  */
 public class MySqlDialect extends AbstractDialect {
@@ -45,7 +46,9 @@ public class MySqlDialect extends AbstractDialect {
 		 */
 		@Override
 		public String getOffset(long offset) {
-			throw new UnsupportedOperationException("MySQL does not support OFFSET without LIMIT");
+			// Ugly but the official workaround for offset without limit
+			// see: https://stackoverflow.com/a/271650
+			return String.format("LIMIT %d, 18446744073709551615", offset);
 		}
 
 		/*
@@ -56,7 +59,7 @@ public class MySqlDialect extends AbstractDialect {
 		public String getLimitOffset(long limit, long offset) {
 
 			// LIMIT {[offset,] row_count}
-			return String.format("LIMIT %d, %d", offset, limit);
+			return String.format("LIMIT %s, %s", offset, limit);
 		}
 
 		/*

spring-data-relational/src/main/java/org/springframework/data/relational/core/dialect/SqlServerSelectRenderContext.java

@@ -33,7 +33,7 @@ public class SqlServerSelectRenderContext implements SelectRenderContext {
 
 	private static final String SYNTHETIC_ORDER_BY_FIELD = "__relational_row_number__";
 
-	private static final String SYNTHETIC_SELECT_LIST = ", ROW_NUMBER() over (ORDER BY CURRENT_TIMESTAMP) AS "
+	private static final String SYNTHETIC_SELECT_LIST = ", ROW_NUMBER() over (ORDER BY (SELECT 1)) AS "
 			+ SYNTHETIC_ORDER_BY_FIELD;
 
 	private final Function<Select, CharSequence> afterOrderBy;

spring-data-relational/src/test/java/org/springframework/data/relational/core/dialect/MySqlDialectRenderingUnitTests.java

@@ -30,8 +30,9 @@ import org.springframework.data.relational.core.sql.render.SqlRenderer;
  * Tests for {@link MySqlDialect}-specific rendering.
  *
  * @author Mark Paluch
+ * @author Jens Schauder
  */
-public class MySqlDialectRenderingTests {
+public class MySqlDialectRenderingUnitTests {
 
 	private final RenderContextFactory factory = new RenderContextFactory(MySqlDialect.INSTANCE);
 
@@ -57,8 +58,9 @@ public class MySqlDialectRenderingTests {
 		Table table = Table.create("foo");
 		Select select = StatementBuilder.select(table.asterisk()).from(table).offset(10).build();
 
-		assertThatThrownBy(() -> SqlRenderer.create(factory.createRenderContext()).render(select))
-				.isInstanceOf(UnsupportedOperationException.class);
+		String sql = SqlRenderer.create(factory.createRenderContext()).render(select);
+
+		assertThat(sql).isEqualTo("SELECT foo.* FROM foo LIMIT 10, 18446744073709551615");
 	}
 
 	@Test // DATAJDBC-278

spring-data-relational/src/test/java/org/springframework/data/relational/core/dialect/MySqlDialectUnitTests.java

@@ -23,6 +23,7 @@ import org.junit.Test;
  * Unit tests for {@link MySqlDialect}.
  *
  * @author Mark Paluch
+ * @author Jens Schauder
  */
 public class MySqlDialectUnitTests {
 
@@ -48,7 +49,7 @@ public class MySqlDialectUnitTests {
 
 		LimitClause limit = MySqlDialect.INSTANCE.limit();
 
-		assertThatThrownBy(() -> limit.getOffset(10)).isInstanceOf(UnsupportedOperationException.class);
+		assertThat(limit.getOffset(10)).isEqualTo("LIMIT 10, 18446744073709551615");
 	}
 
 	@Test // DATAJDBC-278

spring-data-relational/src/test/java/org/springframework/data/relational/core/dialect/PostgresDialectRenderingUnitTests.java

@@ -30,8 +30,9 @@ import org.springframework.data.relational.core.sql.render.SqlRenderer;
  * Tests for {@link PostgresDialect}-specific rendering.
  *
  * @author Mark Paluch
+ * @author Jens Schauder
  */
-public class PostgresDialectRenderingTests {
+public class PostgresDialectRenderingUnitTests {
 
 	private final RenderContextFactory factory = new RenderContextFactory(PostgresDialect.INSTANCE);
 

spring-data-relational/src/test/java/org/springframework/data/relational/core/dialect/SqlServerDialectRenderingUnitTests.java

@@ -30,8 +30,9 @@ import org.springframework.data.relational.core.sql.render.SqlRenderer;
  * Tests for {@link SqlServerDialect}-specific rendering.
  *
  * @author Mark Paluch
+ * @author Jens Schauder
  */
-public class SqlServerDialectRenderingTests {
+public class SqlServerDialectRenderingUnitTests {
 
 	private final RenderContextFactory factory = new RenderContextFactory(SqlServerDialect.INSTANCE);
 
@@ -73,7 +74,7 @@ public class SqlServerDialectRenderingTests {
 		String sql = SqlRenderer.create(factory.createRenderContext()).render(select);
 
 		assertThat(sql).isEqualTo(
-				"SELECT foo.*, ROW_NUMBER() over (ORDER BY CURRENT_TIMESTAMP) AS __relational_row_number__ FROM foo ORDER BY __relational_row_number__ OFFSET 0 ROWS FETCH NEXT 10 ROWS ONLY");
+				"SELECT foo.*, ROW_NUMBER() over (ORDER BY (SELECT 1)) AS __relational_row_number__ FROM foo ORDER BY __relational_row_number__ OFFSET 0 ROWS FETCH NEXT 10 ROWS ONLY");
 	}
 
 	@Test // DATAJDBC-278
@@ -85,7 +86,7 @@ public class SqlServerDialectRenderingTests {
 		String sql = SqlRenderer.create(factory.createRenderContext()).render(select);
 
 		assertThat(sql).isEqualTo(
-				"SELECT foo.*, ROW_NUMBER() over (ORDER BY CURRENT_TIMESTAMP) AS __relational_row_number__ FROM foo ORDER BY __relational_row_number__ OFFSET 10 ROWS");
+				"SELECT foo.*, ROW_NUMBER() over (ORDER BY (SELECT 1)) AS __relational_row_number__ FROM foo ORDER BY __relational_row_number__ OFFSET 10 ROWS");
 	}
 
 	@Test // DATAJDBC-278
@@ -97,7 +98,7 @@ public class SqlServerDialectRenderingTests {
 		String sql = SqlRenderer.create(factory.createRenderContext()).render(select);
 
 		assertThat(sql).isEqualTo(
-				"SELECT foo.*, ROW_NUMBER() over (ORDER BY CURRENT_TIMESTAMP) AS __relational_row_number__ FROM foo ORDER BY __relational_row_number__ OFFSET 20 ROWS FETCH NEXT 10 ROWS ONLY");
+				"SELECT foo.*, ROW_NUMBER() over (ORDER BY (SELECT 1)) AS __relational_row_number__ FROM foo ORDER BY __relational_row_number__ OFFSET 20 ROWS FETCH NEXT 10 ROWS ONLY");
 	}
 
 	@Test // DATAJDBC-278